The present invention relates to a short-wavelength laser element doped with rare earth ions, an optical amplifier doped with rare earth ions, and a wavelength converter doped with rare earth ions, each for use in such fields as optical information processing, optical measurement, and environmental measurement.
Since the 1960s, vigorous research and development has been directed toward rare earth ions with which optical materials are to be doped and, as a result, there have been proposed laser elements doped with rare earth ions which have various arrangements and exhibit laser oscillation at different wavelengths.
In a typical laser element doped with rare earth ions, an optical material doped with rare earth ions is irradiated with light from a flash lamp so that the rare earth ions are excited from the ground state to a higher energy state. With the excitation, the energy level of the rare earth ions shifts from the ground-state level to an upper laser level. Between the upper laser level and a lower laser level is achieved a population inversion of the rare earth ions, resulting in emission of light at a specific wavelength. The emitted light is resonated by reflecting surfaces formed on both ends of the optical material and emitted as a laser beam from the optical material.
By such an excitation technique using the flash lamp, however, light in the higher wavelength region is radiated from the flash lamp so that the optical material is also irradiated with light at such wavelengths that cannot be absorbed by the rare earth ions, resulting in heat generation which adversely affects the optical material and reduces the efficiency with which the radiated light is converted to a laser beam due to a reduced amount of light absorbed by the optical material.
It was not until the 1980s that a solid-state laser, which oscillates in an absorption region specific to the rare earth ions, was used as a pumping source. As the output from a semiconductor laser element used as the pumping source for the solid-state laser is increased, laser elements doped with rare earth ions have been improved in output efficiency as well as reduced in size and price.
As a representative of the laser elements doped with rare earth ions to be excited by semiconductor lasers, there has been known a YAG (Yttrium-Aluminum-Garnet) laser element which uses an AlGaAs semiconductor laser oscillating at a wavelength of 809 nm as the pumping source and enables laser oscillation at a wavelength of 1064 nm. In principle, a conventional laser element doped with rare earth ions to be excited by a laser beam (laser-excited laser element) has been excited only with a laser beam having a wavelength longer than the wavelength of the pumping beam. To be more specific, rare earth ions having absorbed excitation energy are excited and promoted to a given energy level so that oscillation is primarily caused by a radiative transition from a higher energy level, which is lower than the given energy level.
However, there are some types of rare earth ions which exhibit so-called excited-state absorption (ESA) in which the rare earth ions in the higher energy level are excited again to experience a further transition to a still higher energy level because of the long fluorescence lifetime of the metastable energy level. Since the still higher energy level is higher than the energy level of each pumping beam, the laser oscillation is achieved at shorter wavelengths. A short-wavelength laser implemented by such excitation to the still higher energy level based on two-photon absorption is termed an upconversion laser.
As examples of the upconversion laser, there have currently been reported a laser doped with Ho or Er ions which is used as a source of green light at a wavelength of about 530 nm and a laser doped with Tm or Pr ions which is used as a blue light source. Such an upconversion laser requires two light beams at different wavelengths, since it involves primarily two absorption transitions. For example, as reported by Allain et al ("Blue Upconversion Fluorozirconate Fibre Laser," Electronics Letters, Vol. 26, p. 166), blue light at wavelengths of 450 to 480 nm oscillates in a laser element doped with Tm ions when the Tm ions sequentially experience a around-state absorption (GSA) transition and the ESA transition with a light beam at a wavelength of 680nm and with a light beam at a wavelength of 650 nm, respectively. In this case, however, since a Kr laser which is expensive and requires a large-scale element is used as a pumping source, its industrial applications are considerably limited.
There has also been reported an attempt by Grubb et al to obtain radiation of light at a wavelength of 480 nm by exciting Tm ions with light at three different wavelengths produced by a single pumping source ("Upconversion Blue Fiber Laser" Electronics Letters, Vol. 28, p. 1243, 1992).
FIG. 11 schematically shows the structure of a conventional short-wavelength laser element doped with rare earth ions to be excited by light at three wavelengths as a pumping source.
In the drawing are shown: a pumping-source laser element 800; a semiconductor laser element 801; a solid-state laser element 802 composed of a YAG crystal doped with Nd ions (hereinafter referred to as Nd:YAG solid-state laser element); first and second end mirrors 803 and 804 of the Nd:YAG solid-state laser element 802; a prism 805; a power source 806; a lens system 807; a fluoride-based optical fiber 808 doped with Tm ions; an incident mirror 809 of the optical fiber 808; an emitting mirror 810 of the optical fiber 808; a pumping beam 811 outputted from the pumping-source laser element 800; and a laser beam 812 at a wavelength of 480 nm emitted from the conventional short-wavelength laser element doped with rare earth ions.
In the conventional pumping-source laser element 800, light emitted from an array of about twenty AlGaAs-based semiconductor laser elements 801 irradiates the Nd:YAG solid-state laser element 802. The laser beam emitted from the Nd:YAG solid-state laser element 802 is divided by the prism 805 and only the laser beam in the 1100 nm wavelength region is reflected by the first end mirror 103 and returns to the Nd:YAG solid-state laser element 802. Thus, the first end mirror 103 and the second end mirror 104 constitute a resonator. The pumping-source laser element 800 emits the pumping beam 811 at a wavelength of about 1100 nm, which arrives at the optical fiber 808 to be incident thereon. The pumping beam 811 at a wavelength of about 1100 nm incident on the optical fiber 808 is absorbed by the Tm ions with which the optical fiber 808 has been doped.
FIG. 12 shows energy levels of the Tm ions in the conventional short-wavelength laser element doped with rare earth ions. Specifically, the drawing shows the energy levels of the Tm ions excited by the pumping beam 811 in the 1100 nm wavelength region. In the drawing, the vertical axis represents the energy level of the Tm ions in unit cm.sup.-1 (Kayser). There are also shown: respective absorption transitions 852, 853, and 854; a radiative transition 855; and a non-radiative transition 856.
First, the Tm ions absorb the pumping beam and are thereby excited to undergo the GSA transition 852 from the ground-state level .sup.3 H.sub.6 to the level .sup.3 H.sub.5. Subsequently, the Tm ions in the level .sup.3 H.sub.5 undergo the non-radiative transition 856 to the level .sup.3 F.sub.4. The Tm ions in the level .sup.3 F.sub.4 further absorb the pumping beam and are thereby excited to sequentially undergo the ESA transition 853 to the level .sup.3 F.sub.3 and again the non-radiative transition 856 to the level .sup.3 H.sub.4. The Tm ions in the level .sup.3 H.sub.4 level further absorb the pumping beam and are thereby excited to undergo the ESA transition 854 to the level .sup.1 G.sub.4, which is the upper laser level. The light emitted on the radiative transition 855 from the level .sup.1 G.sub.4 to the ground-state level .sup.3 H.sub.6 propagates the core of the optical fiber 808, resonated by the incident mirror 808 and emitting mirror 810 of the optical fiber 808, and then emitted as the laser beam 812 through the emitting mirror 810.
As an optical fiber amplifier doped with rare earth ions, there has been known one which primarily amplifies light at wavelengths for optical communication, i.e., wavelengths in the vicinity of 1300 nm or 1550 nm.
As a wavelength converter for converting the wavelength of light, there has been known one which has an optical waveguide for converting the wavelength of transmitted light to that of second harmonic light.
If upconversion is to be effected by a single pumping-source element, therefore, it is necessary to cause excitation with three pumping beams of light at three different wavelengths consisting of one pumping beam for the GSA and two pumping beams for the ESA and having overlapping absorption regions.
In the conventional short-wavelength laser element and optical amplifier each doped with rare earth ions, laser beams at three wavelengths of 1112 nm, 1116 nm, and 1123 nm are emitted from the pumping-source laser element 800 so as to effect one GSA transition and two ESA transitions in the Tm ions, thereby accomplishing upconversion.
However, if three absorption transitions are effected by the single pumping-source element, the efficiency with which the pumping beam causes laser oscillation is reduced.
Moreover, since the wavelengths of the three pumping beams are different from 1064 nm, which is the normal wavelength of the Nd:YAG solid-state laser, some adaptations are required to cause laser oscillation at the foregoing three wavelengths, which renders the pumping-source laser element complicated and bulky.
Furthermore, since the pumping-source laser element 800 has used the plurality of semiconductor lasers 801 therein, the short-wavelength laser element and optical amplifier each doped with rare earth ions are increased in scale, which restricts their industrial applications and increases their prices.
Although the Tm ions have been excited with the three pumping beams at the three wavelengths having overlapping absorption regions on one GSA transition and on two ESA transitions, their respective absorption coefficients are low in the overlapping absorption regions. Specifically, the peak wavelength on the GSA transition is in the vicinity of 1210 nm, whereas the peak wavelength on the second ESA transition (absorption transition from the level .sup.3 F.sub.4 to the level .sup.1 G.sub.4) is in the vicinity of 1150 nm. It follows that a difference of 60 nm or more exists between the two peak wavelengths and hence high pumping power is required disadvantageously to achieve an oscillation threshold, while the efficiency with which the pumping beam causes laser oscillation is low.
The semiconductor laser currently used cannot provide sufficient power required by the pumping beam in the 1100 nm wavelength region to effect efficient upconversion in the Tm ions, since the semiconductor laser currently used emits a laser beam in the 1200 nm wavelength region. Although it is possible to obtain the pumping beam at a wavelength of 1100 nm from a semiconductor laser of quantum-well structure, the quantum-well structure should have about 5% of distortion, which impairs the characteristic of a semiconductor laser used as the pumping source.
Although an upconversion laser using an optical fiber is designed such that a pumping beam propagates the interior of the optical fiber in a single mode, a laser beam propagates the interior of the optical fiber in a multimode since the wavelength of the laser beam is shorter than that of the pumping beam, which considerably increases a propagation loss in a high-order mode.
There has also been known a method which uses an optical fiber with a large numerical aperture in order to increase the power density of the pumping beam in the optical fiber. In the case of using an upconversion laser, however, the pumping beam is not used for upconverslon efficiently if the mode of the pumping beam is considerably different from the mode of a signal beam.
In the wavelength converter having the optical waveguide for converting transmitted light to second harmonic light, some restrictions are placed on the relationship between the wavelength of the light prior to conversion and the wavelength of light after conversion, while the conversion efficiency is low.